Historically inorganic white pigments, such as hydrated or anhydrous siliceous minerals including metal silicates, sodium aluminosilicates, hydrated aluminum silicates, e.g., clays, have found increasing uses in many industries. For example, such pigments are employed in paper coating compositions, as fillers for paper, paints, inks, etc. In addition to being used as fillers, these inorganic pigments are often used as color-imparting fillers in papers, paints, inks, etc., and as color-imparting reinforcing pigments or fillers in elastomers and other polymeric materials.
In the process of dyeing inorganic pigments, it is important to fix or chemically attach dyes onto the pigment particles. In this regard, it is usually necessary to treat pigment particles in a manner so as to improve the surface affinity of the pigment particles to the dyestuffs utilized.
Several methods of improving the surface affinity of various inorganic pigment particles to dyestuffs have been reported. U.S. Pat. No. 3,834,824 to Grillo, teaches a process for manufacturing surface modified inorganic pigments. The process according to this patent includes adding amino organosilanes to an aqueous dispersion of an inorganic pigment whereby the inorganic pigment is contacted with the amino organosilane and reacts so that the surface of the inorganic pigment is modified in a manner to increase its affinity for dyes.
U.S. Pat. No. 3,545,909 to Gagliardi, teaches aminoalkyl silicone coloring assistants that may be applied in the form of a pre-treatment to condition inorganic pigment substrates for a subsequent coloring operation. This pre-treatment with the aminoalkyl silicone coloring assistants induces dyeable sites onto normally non-affinitive materials so that they may accept conventional anionic type organic dyes.
U.S. Pat. No. 4,084,983 to Bernhard et al, teaches a process for producing colored lustrous pigments in which a firmly adhering coating of aluminum hydroxide is first applied to pigment substrates. The aluminum ions contained in the coating are then reacted to form a firmly adhering dyestuff layer. In this manner, the substrate is coated with a starting material which is convertible into a colored layer.
U.S. Pat. No. 4,543,128 to Troesche et al, teaches a process for dyeing inorganic pigments that can be used as fillers. According to this patent, pigments are dyed with polycationic dyestuffs in a process that involves providing a colored composition of an aqueous paste or dispersion of a white pigment with a water-soluble polycationic dye. Optionally, a conventional fixing agent or metallized cationic dye may be used in said process. Colored pigments are produced by the indicated treatment process, but this art does not teach the use of cationic dyes for the specific purpose of improving pigment whiteness.
U.S. Pat. No. 4,566,908 to Nakatani et al, teaches a process of producing an azoic pigment including a silica core with a coating of amino or polyazoic dye chemically bound to the surface of the silica core through an amino silane coupling agent.
U.S. patent application Ser. Nos. 07/427,340 (now U.S. Pat. No. 5,106,421) and 07/427,342, filed on Oct. 27, 1989, teach mineral based materials, such as kaolin, that have been colored with a variety of dyes. In general, these applications disclose that direct dyeing is not used. Materials identified as anionic chemical fixatives are used to insolubilize basic dyes onto the surface of the inorganic pigment and cationic chemical fixatives are used to insolubilize acid or direct dyes. Various resinous polymers, methylolamide polymers and/or quaternary ammonium polymers are useful as cationic fixatives, while resorcinol formaldehyde resins, polyacrylates and related materials can function as anionic chemical fixatives. These applications furnish considerable detail concerning the dyed pigments and the preparation thereof.
U.S. Pat. No. 3,950,180 to Kato does teach the direct introduction of basic water-soluble dyestuffs onto zeolites and montmorillonite. Such materials have fairly large cation exchange capacities in the range of 60 to 400 meq. per 100 gm of material. These materials must be pretreated with acids and/or bases, or so treated during the dyeing process to give the surface the desired cation exchange capacities. In the specification of the patent, column 1, lines 9-28, it is stated that:
"In general, clay minerals are used as fillers for extenders or lake pigments. Many of such clay minerals consist of mainly kaolinite, talc or pyrophyllite and are useful in making them inexpensive in cost. On the other hand, pigments manufactured by making organic dyestuffs adsorbed on said clay minerals are also used. In these pigments, however, the organic dyestuffs are adsorbed only on the surface of these clay minerals, the adsorbed quantity thereof being less than 1%. Namely, these clay minerals possess no cation between the crystal layers or in the three dimensional network structure thereof, consequently color development in these clay minerals is due to a mere physical adsorption of the dyestuffs on the surface thereof. The cation exchange capacity of these clay minerals is caused by silanol radicals existing at the ends or fractures of the crystals, and is only 5-10 meq per 100 g. Therefore, these pigments have a weak coloring power and a thermal resistance lower than that of the dyestuffs adsorbed and consequently are limited in uses."
The clear teaching of the reference is that direct dyeing of clays, such as kaolin, would not be expected to lead to commercially useful colored products.
Kato also teaches that the amount of basic dyestuff needed is 0.01 to 50 wt. % of dried inorganic material. The implication of the patent is that unless the zeolite or treated montmorillonite can react with at least that amount of basic dye, an unsatisfactory, weakly dyed unstable pigment will be obtained.
What the prior art does not recognize is that the inability of an inorganic pigment, such as kaolin, to be strongly dyed because of its very low cation exchange capacity can, in fact, be advantageous. It can be utilized to react said inorganic pigments with very small amounts of basic dye, the resulting weakly dyed materials having considerable utility.
U.S. Pat. No. 4,661,164 to Severinghaus reveals that the optical whiteness properties of various mineral fillers, like clay or limestone, can be improved by a tinting strategy that involves adding "non-bonding" blue pigments which are intimately intermixed therein. The blue, water insoluble pigment of choice in Severinghaus' process is Ultramarine Blue. This tinting process strictly relies on producing an intimate physical mixture of pigments rather than bonding water-soluble dyes to the surface of an inorganic pigment. Hence this prior art suffers from several disadvantages, including: 1) loss of final pigment brightness; 2) absence of means for neutralizing the greenish-white tint that is produced; 3) possibility of tinting pigment separating from mineral; and 4) optical tinting inefficiencies relative to dyes (see Example 5).